Effect of thermal treatment on K3PW12O40 for cyclohexene oxidation reaction to adipic acid

Abstract

The use of heteropolysalts in the cyclohexene oxidation reaction has shown good results, but there is still a significant solubility of this catalyst in the aqueous phase of the reaction medium which is not discussed in most articles dealing with this subject. A thermal treatment, such as calcination step, can significantly affect the properties of a catalyst, including its solubility, sedimentation and acidity. These effects are very important in batch reaction processes and frequently neglected. Thus, the main objective of this work was to study the effect of different calcination temperatures (200 and 600 °C) on the solubility and the acidity of Keggin-type catalysts (K3PW12O40). In addition, the effect of reaction temperature (65, 75 and 85 °C) on the conversion and distribution of products in the oxidation of cyclohexene with these two catalysts was analyzed. The heteropolysalts were characterized using XRD, Raman spectroscopy, N2 physisorption, SEM-EDS, acid-base titration and NH3-TPD. XRD and Raman results showed that the catalysts synthesis was carried out successfully and the thermal treatments did not affect the polyanion structures, even at the highest temperature (600 °C). When compared to the catalyst treated at 600 °C, the KPW-200 catalyst showed greater solubility in the reaction medium. However, it achieved 100% conversion and the highest yield of adipic acid (89%) at 75 °C, in addition to the highest density of acid sites. Tests were performed using the aqueous phase of the post-reaction solution to evaluate the occurrence of homogeneous catalysis under the same conditions. The results showed that, despite the solubilization of the catalyst, there was no adipic acid formation, only of intermediate compounds. Thus, the contribution of homogeneous catalysis in the synthesis of adipic acid is discarded, and the acidity of the catalyst seems to be the determining parameter in the formation of this product. Acidity can influence both the oxidation and hydrolysis steps present in the reaction mechanism and can justify these results.